Engines can be coupled to emission control devices, such as catalytic converters, to reduce exhaust emissions. However, these devices can become contaminated with sulfates, for example. In order to remove these contaminates, the temperature of the emission control device is raised significantly and a near stoichiometric air-fuel ratio is provided that alternates, or oscillates, around stoichiometry (between lean and rich).
One type of engine exhaust system routes all of the engine cylinders into a single exhaust path. One approach for raising temperature of such an exhaust path sequentially operates some cylinders lean, and then some rich. Such an approach is described in DE 19923481, for example. In such an approach, the ending of both the lean and rich mixture air-fuel ratio is controlled via a sensor downstream of the device.
The inventors herein have recognized a disadvantage with such an approach. In particular, when the device is completely filled and then completely purged of stored oxidants, a large portion of the heat generated in the rearward section of the catalyst is simply lost through the exit of the device via the exhaust flow. I.e., the exhaust flow carries a significant portion of the generated heat away from the catalyst. This creates an uneven temperature profile in the device, resulting in uneven catalyst degradation and uneven sulfur removal. Furthermore, such an approach requires a significant breakthrough of reductants (to be detected by the downstream sensor), thereby resulting in potentially increased emissions.